1. Field of the Invention
The present invention relates to a recording apparatus and a recording control method, and more particularly to a recording apparatus and a recording control method wherein ink droplets are ejected onto a recording medium by using an ink jet recording head, for example, to record an image in accordance with the multi-pass recording technique.
2. Description of the Related Art
As color recording techniques, there are known at the present, for example, the thermal transfer type in which recording is performed by transferring ink on an ink ribbon to a recording medium, e.g., a sheet of recording paper, with thermal energy, the ink jet recording type in which recording is performed by ejecting ink droplets to be deposited on a recording medium. Above all, attention is focused on the ink jet recording type because it is based on the non-impact recording technique generating no noise during the recording operation, permits high-speed recording, and enables an image to be recorded on plain paper without a need for a special fixing process.
In a serial printer wherein an image is completed by reciprocally scanning a recording head, which employs the above-stated ink jet recording technique, plural times while ink is ejected from the recording head toward a recording medium, e.g., a sheet of recording paper, the accuracy of feed of the recording medium and the ejection ability of the recording head may adversely affect recording quality. For example, unevenness in density of a recorded image may result from poor accuracy in feed of the recording medium, and undesirable streaks, that is, banding may result from variations in ejecting operation of the recording head.
To solve the above drawback, there has been proposed a multi-pass recording technique wherein the same area of a recording medium is scanned multiple times by different portions of a recording head to completely record an image in that area of the recording medium. Specifically, in such a multi-pass recording technique, a mask matrix of predetermined size is applied to original image data to compute a logical AND of the two for thinning the number of pixels (picture elements) to be recorded per scan, and ink is ejected from different nozzles of the recording head for each scan. In addition, the distance over which the recording medium is fed per unit time is set in the recording operation to be shorter than the recording width of the recording head. This holds down unevenness in density and undesirable streaks which would otherwise be caused by the fact that variations in ejecting ability between the nozzles of the recording head are concentrated on a particular portion of the recording medium, as well as by poor accuracy in feed of the recording medium.
FIGS. 12A and 12B show examples of a mask matrix of 8 dots.times.8 dots for use in a recording technique (2-pass recording) with which image recording is completed by scanning the same area of a recording material two times by different portions of a recording head. FIG. 12A represents a pattern of the mask matrix (hereinafter referred to as "MASK 1") used for the first scan (first pass), and FIG. 12B represents a pattern of the mask matrix (hereinafter referred to as "MASK 2") used for the second scan (second pass). In each of the MASKs, a pattern of 4 bits.times.4 bits shown surrounded by wide lines is repeated.
Assuming, for example, that the number of nozzles of the recording head is 256 and the nozzles are arrayed in a line in the direction of feed of the recording medium, multi-pass recording using the mask matrixes shown in FIGS. 12A and 12B is performed under recording control described below.
Initially, in a first-pass recording cycle, the logical AND of original image data corresponding to the 256 nozzles and the MASK 1 is computed and upper-half data of the computed result is output, as data for the first pass, to the 128 nozzles in the lower half of the recording head. As a consequence, the recording is made by using the lower-half 128 ones of the total 256 nozzles of the recording head.
Then, the recording medium is fed through a distance corresponding to the recording width covered by a half, i.e., 128 nozzles, of the total 256 nozzles of the recording head. Subsequently, the logical AND of the original image data corresponding to the 256 nozzles and the MASK 2 is computed and the computed result is output, as data for the second pass, to the recording head. As a consequence, the recording is made in the recording width corresponding to the total 256 nozzles by using the image data masked by the MASK 2. The recording of the first pass and the recording of the second pass are thus superposed in the upper half of the total recording width of the recording head.
Further, the recording medium is fed again through a distance corresponding to the recording width covered by a half, i.e., 128 nozzles, of the total 256 nozzles of the recording head. Subsequently, the computed result for the lower-half 128 nozzles obtained from the above-stated logical AND operation between the original image data corresponding to the first set of 256 nozzles and the MASK 1 and the computed result of the logical AND operation between those of the original image data corresponding to the next set of 256 nozzles, which are associated with the lower-half 128 nozzles thereof, and the MASK 1 are output, as data for the third pass, to the 256 nozzles of the recording head. As a consequence, a half of the image corresponding to the original image data for the first set of 256 nozzles, which results from masking it by the MASK 1 and using the upper-half 128 nozzles of the recording head, and a half of the image corresponding to the original image data for the next set of 256 nozzles, which results from masking it by the MASK 1 and using the lower-half 128 nozzles of the recording head, are both recorded. The recording of the second pass and the recording of the third pass are thus superposed in the upper half of the total recording width of the recording head.
Through the above process, the recording of the image with a recording width corresponding to the 256 nozzles of the recording head is completed.
Meanwhile, when the mask matrices shown in FIGS. 12A and 12B are used, the same nozzles are employed at the same timing at a 4-dot period in both the main scan direction in which the recording head is moved and the sub-scan direction in which the recording medium is fed. Suppose now that a printer has resolution of 300 (in the main scan direction).times.300 (in the sub-scan direction) DPI and an A4-size sheet of recording paper is used as the recording medium with its long side extending in the sub-scan direction, the same nozzles are employed at the same timing repeatedly (2480.div.4) times because the number of recording dots in the main scan direction is about 2480 dots. Accordingly, variations in ability of the nozzles are repeated at the 4-dot period, which makes it difficult to perfectly eliminate unevenness in density and undesirable streaks. In addition, because the mask pattern is fixed, the effect of the multi-pass recording is not obtained if recording data is synchronized with the mask pattern.
To solve the above problem, in Japanese Patent Laid-Open No. 7-52390 (U.S. patent application Ser. No. 266,498), the inventors proposed a method of using a mask matrix with a mask pattern distributed at random in an area of predetermined size (e.g., 2400.times.8 dots). In the proposed method of using such a mask (random mask), random values in predetermined number of bits are stored in a ROM beforehand, and the random values are read out of the ROM depending on the number of recording passes to create a pattern of each random mask on a RAM. Using the random mask makes the period of ink ejection irregular in both the main scan/sub-scan directions, and hence contributes to preventing unevenness in density and undesirable streaks. In addition, since the mask pattern is random, the possibility that recording data may synchronize with the mask pattern is very low.
The inventors however found that even the above-proposed recording method on the basis of the multi-pass recording technique using the random mask raises a problem described below for a printer having recording resolution as high as 1200.times.600 DPI, for example.
Specifically, the positions where ink droplets ejected from the recording head are deposited on the recording medium are shifted in units of each pixel of 1200.times.600 DPI corresponding to the mask resolution, and the shift appears on a recorded image as unevenness in density with high spatial frequency which is easily recognized by human eyes. Elimination of such unevenness in density requires not only highly accurate mechanical control relating to the movement of the recording head and the feed of the recording medium in both the main scan/sub-scan directions, but also high standards in ejection ability of the recording head as well. But there are limits to improving the accuracy of the mechanical control and the ejection ability of the recording head. Even if an improvement is realized to some extent in those points, the technique permitting such an improvement necessarily pushes up cost of the entire recording apparatus.